Lifting magnet controller



March 26, 1940.

A. L. WARD LIFTING MAGNET CONTROLLER Filed Nov. 10, 1937 ARTHUR L. WARDHIS ATTORNEY.

Patented Mar. 26, 1940 UNITED STATES PATENT OFFICE to The ElectricController 8 Mann! will! Company, Cleveland, Ohio, a corporation of OhioApplication November 10. 1031, Serial m. 11am iiOlaims.

This invention relates to a control system for inductive devices, andmore particularly to a protective control system for lifting magnets,power solenoids and the like.

Unless a voltage limiting protective device is used, the inherentself-inductance of a highly inductive winding will cause a high voltageto be induced consequent upon disconnection of the winding from thesource of supply. This large induced voltage-results in destructivearcing at the switching contacts and also causes a high voltage strainon the insulation of the winding itself and the conductors connectedthereto.

In some applications of inductive windings it is common practice toconnect a resistance across the terminals of the winding at the time ofinterruption. The use of such a resistance limits the discharge voltageand prevents destructive arcing or high voltage strains. However, theuse of a resistance in this manner results in aslow decay of magneticflux which is very undesirable in lifting magnet operation because ofthe resultant delayed dropping of the load.

To reduce the flux quickly and still prevent damaging induced voltages,it has been found expedient to apply reverse voltage to the windingafter it has been disconnected from the source of supply. Even if thisis done, it is also necessary in order to prevent the induction ofdestructive voltages, to provide a discharge circuit at the instant ofthe removal of the energization voltage. A further improvement in thecontrol of such inductive devices is disclosed in United States ReissuePatent No. 20,274, issued on May 10, 1938, to D. C. Wright, wherein isdescribed a control system providing for the connection of a liftingmagnet to a reverse power source prior to the removal of energizingpower connections and which permits the magnet to discharge into thesupply source through protective resistance, which resistance alsoserves to prevent a short circuit across the supply circuit during theswitching operation. 7

Amongvarious prior control systems with which the present invention maybe used is one such as described in the Wright patent which operatesvery satisfactorily and has eliminated nearly all of the defects of theearlier prior art devices. But one of the most common troublesexperienced in the use oi any lifting magnet controller has not beeneliminated. This trouble is the failure of the discharge resistors andresultant open-circuit condition. One of the reasons for prevalence ofthis trouble is that the discharge resistors are subjected to suddenimpulses of very high currents which heat them to hightemperaturessothat theyhavetobeplacedin anexposed position in order topermit cooling. Furthermore, they are usually mounted upon thecranewhich is handling the magnet and are thus subiected to very severevibrations. This alternate heating and cooling, exposure to theelements, and vibration make it impossible to design practical resistorswhich will not break after a certain length of time. Another commonreason for the failure of discharge resistors to limit the dischargevoltage is that the conductors required to connect the controller itselfto the resistors, which must be mounted in an exposed place, are oftennecessarily long and thus exposed to mechanical damage.

Heretofore, if a discharge resistor became opencircuited for any reasonwhatsoever, it was possible to continue using the magnet, withconsequent deleterious arcing and possibility of damage to the magnetitself. In such cases, if the operator would refrain from reporting thedamage because it would tend to delay him in completing his task, or forother reasons, or if the contactors were enclosed in a cabinetpreventing the trouble from being quickly detected, the contacts on theswitch supplying power to the magnet would be badly damaged ordestroyed, or the insulation of the magnet would be punctured and thewindings short-circuited. Although signal lights or alarm bells could beused to inform those in authority that such a break had occurred, theimpracticability and additional expense of such complications in thecrowded cabs of cranes using lifting magnets is readily apparent.

It is therefore an object of this invention to provide a lifting magnetcontrol system employing resistors and wherein energization of themagnet is impossible when a discharge resistor is open-circuited.

It is a further object of this invention to provide an improvedprotective lifting magnet control system of this character whichrequires no extra equipment to render impossible the energization of themagnet as long as a discharge resistor is open-circuited.

A more specific object is to provide such a system which employsswitching means for closing the circuit to the magnet and which meansare so series connected with the resistors that the switching means arerendered inoperative to close the circuit when the resistors areopen-circuited.

Other objects and advantages will become epparent from the followingspecification, wherein reference is made to the drawing, in which:

Fig. 1 is a full diagrammatic diagram of a' A magnet winding 7 of anelectromagnet I has terminals 9 and N which are connected to theconductors 4 and 5 respectively when the switch 8 is in a closedposition. An electro-magnetic switch having an operating coil llw,normally-open contacts Ho and llb, and a biasing spring Is, is arrangedto provide reverse voltage connections for the magnet winding 1 byconnecting terminals 9 and ID to conductors 5 and 4, respectively,through conductors l5 and I4 and resistors 35 and 34, respectively.

A master switch 18 controls the operation of the switches i and H. Themaster switch 18 is preferably of the snap action type and is shown ashaving a toggle action spring Its, having one end connected to a movablepart of the switch l8 and the other end stationary so that when theswitch 18 is moved past a dead center position, the spring lBs willcause the switch to rapidly move to one of its closed positions. Theswitch H has a lift button I2, 9. drop button II, and stationarycontacts I9, 20, 2| and 22. When the drop button i3 is operated, aconnector |Ib electrically connects the contacts 2| and 22, and when thelift button |2 is operated, a connector |8a electrically connects thecontacts I! and 2|, and the connector i8?) is removed from the contacts2| and 22. The connectors Ila. and llb move simultaneously so that onlyone of them is completing a circuit when the master switch it is in one01 its closed positions.

The contacts I! and 2| are connected to the conductor H at a point 24,and the contact 20 is connected to one terminal of the coil he of theelectromagnetic switch I. A conductor 21 leads from the other terminalof the coil Iw to the conductor II at a point 26. The contact 22 isconnected through the contacts 6c of the switch I to a terminal 01' thecoil llw; the other terminal of the coil ||w is connected through thecontacts Id of the switch I to the conductor 5. Another connection fromthe terminal of coil llw which is connected to the contacts d is througha resistor 4| to the conductor II at the point 2|. A resistor 4| shuntsthe contacts 6c and completes a circuit from the contact 22 to theterminal of the coil ||w which is connected to the contacts 40. Thepurpose of the resistors 4I and 4| is to reduce to a safe value thevoltage applied to the coil llw when the magnet I is discharging.

To provide an adjustment of the operating characteristics of the switchI, an adjustable resistor 31 having an adjustable connector II isconnected in parallel with the coil an. A fixed resistor 28 is connectedin series with the adjustable resistor 31.

The control system operates as follows:

If the knife switch I is closed so as to apply power from the source and2 to the conductors 4 and 5, and if the lift button I2 is operatedconnect contacts l8 and 2| through the connector Its, an operativecircuit is set up for the coil 410 from the conductor 4 through theresistor 34 to the point 24 on the conductor I4, through the masterswitch II by means of connector lIa to the coil B10, and by means of theconductor 21 to the point 2! on the conductor II, and thence through theresistor II to the conductor 6. Energizatlon of the coil Iw operates theswitch I to its closed position and power is supplied to the magnetwinding 1 through a circuit which extends from conductor 4, the contactIa, the terminal 9, the winding I, the terminal II, and the contact 5bto the conductor I.

When it is desired to deenergise the magnet winding 1, the drop buttonII is operated to connect contacts 2| and 22 by means of connector IIb,and simultaneously connector IIa is removed from the contacts II and 2I.The coil Ilw of the switch II is then energized through a circuit whichextends from the conductor 4, through the resistor 24 to the point 24 onthe conductor 4, the connector |Ib, through the now closed contacts Ioof switch I, the coil Hu: and through the now closed contacts Id toconductor 5. g

It is a well-known fact that the contacts of an electromagnetic switchclose more quickly upon energlzation of its operating coll than thecontacts of a similar switch open upon deenergization of its operatingcoil. Thus the switch closes before the switch I opens and a dmchargecircuit is then provided through the power lines before the magnet isdisconnected therefrom. If the natural time 0! opening oi the switch Iis not longer than the natural closing time of the switch any of.several well-known arrangements may be used either to speed up theclosing of the switch H or to slow down the opening of switch I, orboth. For instance, a short-circuited winding may surround the core ofthe switch I to delay its demagnetization, or the coil llw might bedesigned to provide an extremely large closing force. The dischargecircuit extends from the terminal II of the magnet winding I, throughthe contacts MD, the conductor I4, the resistor 24, the conductor 4, theknife switch I, the supply source and 2, the knife switch I, theconductor I, the resistor II, the conductor II, and the contacts Ila tothe terminal I 'of the winding 1. when the switch I opens, the dis-'charge voltage of the magnet is opposed to the supply line voltage andwhen the discharge voltage, which may be twice or three times as much asthe supply voltage at the beginning of the discharge, has reduced to avalue equal and opposed to the applied voltage, the current in themagnet winding 1 is zero. Current now begins to flow in the reversedirection through the magnet winding 1, over a circuit which extendsfrom the conductor 4, the resistor 24, the conductor l4, the contactsllb, the terminal II, the winding 1, the terminal I, the contacts Ila,the conductor II, and the resistor II to the conductor I. The currentnow begins to build up in the reverse direction in the magnet winding 1and tends to cause the remaining flux in the magnet to decrease to azero value.

The original circuit to the coil llw through the contacts Ic and 6d ofswitch I was interrupted when the switch I was opened, but the coil ||wremained energised due to the discharge voltage of the magnet over acircuit which extends from the terminal I, the contacts Ila, the

' conductor lithe point 20, the resistor 40, the

coil llw, the resistor 4|, the connector lib, the point 24, theconductor l4, and the contacts ||b, to the terminal l0. when thedischarge voltage reaches a predetermined value due to the reversecurrent, the operating coil to will be sufliciently deenergized topermit the switch II to open, thereby cutting 08 the current at themoment the magnet 8 is de-magnetized to the required degree.

The adjustable resistance 31 is provided in the circuit of the coil ||wto permit control of the value of reverse current which will allowoperation of the switch II to its open position. By use of theadjustable resistance 31 the reverse current can be set to give the bestdrop on any material which the magnet 8 might be lifting and can also beadjusted for changes in line voltage. The purpose of the fixedresistance 39 is to prevent too great a current inrush through thauxiliary contacts 6c and 6d of the switch 6 upon the closure of theswitch II in case the adjustable resistance 31 should be all removedfrom the circuit by means of the adjustable connector 38.

When the magnet 8 is discharged and the drop button I3 01 the masterswitch l8 closed, 9. current from the source I and 2 flows through thecoil Ilw over a circuit which extends from the conductor 4, through thermistor 34 to the point 24 on the conductor H, the connector lBb, theresistor 4|, the coil Hw, the resistor 40, the point 25 on the conductorl5, and the resistor 35 to the conductor 5. The current in this circuitis not suflicient to close the switch due to the high ohmic value of theresistors 40 and 4| in relation to the voltage of the source and 2. Dueto the fact that the resistors 40 and 4| are also included in theoperative circuit previously traced for the coil ||w when the magnet lis discharging, it is obvious that an open circuit in either of theresistors 40 or 4| would prevent proper operation of the switch II andconsequent destructive arcing or other damage, as is true of prior artcontrollers which employ resistors for a similar purpose. However, it isto be noted that no current flows through the resistors 40 and 4| whenthe switch 6 is closed, thus greatly reducing the heating thereof andrendering them less likely to break and cause an open circuit.

If the lift switch 6 was permitted to close, as was the case heretofore,even though the discharge resistor 34 or 35 might be burned out, themagnet winding 1 would be energized. In such case, the closing of thedrop switch would be of no effect, and upon the opening of the liftswitch 6 a voltage of some ten times the line voltage would be inducedin the winding I to cause a high voltage strain on the insulation and adestructive arc to be formed at the contacts of the switch 8.

By virtue of the connection of the operating coil 8w of the switch 6 tothe conductor H at the point 24, and to the conductor l5 at the point25, no current can flow through the coil 6w from the source and 2, andthe switch 8 cannot be operated to energize the magnet winding 1, unlessboth resistors 34 and 38 are in good condition. In event that either orboth of these resistors should burn out or become disconnected for anyreason, no current can be supplied to the operating coil 610 andconsequently no power can be applied to the magnet winding 1. Thus it isimpossible for the operator of the magnet to use the magnet unless thedischarge resistors which limit the discharge current to a safe valueare in condition to perform their proper function.

Having thus described my invention, I claim:

1. In a control system for a lifting magnet, a source of power,switching means for connecting the winding of the lifting magnetdirectly to the source of power, resistance means, reverse switchingmeans for connecting the winding to said source through said resistancemeans for applying reverse power to said winding, electromagneticoperating means for said first mentioned switching means, saidelectromagnetic operating means being connected to the source of powerin series with said resistance means, whereby said electromagnetic meansis rendered inoperative when said resistance means is open-circuited.

2. In a control system for a lifting magnet to be connected to anddisconnected from a source of power, said system including dischargeprotective means for preventing the occurrence of high induced voltagesupon deenergization of the magnet resulting from disconnection of themagnet from the source of power, electromagnetic switching meansselectively operable to connect and disconnect the magnet to and fromthe source of power, means connecting said discharge protective means tothe magnet, magnetic operating means energizable from the source ofpower for operating the switching means to connect the magnet to thesource of power and being connected to the source of power through saiddischarge protective means, whereby the magnetic operating means cannotoperate the switching means to connect the magnet to the source of powerwhen said discharge protective means is open-circuited.

3. In a control system for a lifting magnet having an operating coil tobe connected to and disconnected from a source of power, a pair ofcontacts connected to two opposite terminals of the coil, respectively,and to said source in series with the coil, a resistor connected to oneof said terminals of the coil and forming a parallel circuit with thecoil and with the one of said contacts which is connected to the otherof the said two terminals of the coil, the parallel circuit being inseries with the other contact of said pair across the source of power,another resistor connected to said other of said terminals of the coiland forming a parallel circuit with the coil and with the contact whichis connected to the said one of said terminals of the coil, the secondparallel circuit being in series with the said one of said contactsacross the source of power, switching means for opening both of saidcontacts concurrently, whereby, when the contacts are open, the coilremains connected to said source of power in series with said resistors,thereby reversing the power applied to the coil, and electricallyoperated means operable when energized for closing said contacts andholding said contacts in closed position and operable when unenergizedfor opening said contacts, and said electrically operated means beingconnected to the source of power in series with both of said resistors,whereby the switching means are rendered inoperative for closing thecontacts when either one, or both, of said resistors are open-circuited.

4. In a control system for a lifting magnet, switching means for closinga circuit through said magnet from a current supply source,electromagnetic operating means for closing said switching means, meansfor dissipating the inductive discharge of the magnetic field while themagnet is being deenergized, said dissipating means being connected tosaid source, and a discharge circuit connecting the magnet to saiddischarge dissipating means, said electromagnetic operatlng means beingconnected to said source through a point on said discharge circuitbetween said discharge dissipating means and said magnet, whereby saidswitching means cannot be operated if said discharge circuit isopen-circuited between said point of connection and said source.

5. In a control system for a lifting magnet, resistance means, switchingmeans to complete a circuit for said magnet exclusive oi said resistancemeans, switching means to complete a.

circuit for said magnet inclusive of said resist v ance means, saidsecond-named switching means being in said second-named circuit betweenthe magnet and said resistance means, and electro-- magnetic operatingmeans for the first-named switching means, said operating means beingconnected for energization to a point on said second-named circuitbetween said second-named switching means and said resistance means.

6. In a control system for a lifting magnet, switching means for closinga circuit through said magnet from a current supply source,electromagnetic operating means or closing said switching means, meansfor dissipating the inductive discharge of the magnetic fleld while themagnet is being deenergized, said dissipating means being connected tosaid source, and a discharge circuit connecting the magnet to saiddischarge dissipating means, said electromagnetic operating means beingconnected for energization only in response to currents flowing in acircuit completed by said discharge dissipating means, whereby saidswitching means cannot be operated it said discharge dissipating meansis opencircuited.

7. The combination with a control system for a lifting magnet and asource 0! power therefor,

including switching means normally operable to connect the liftingmagnet to the source of power. and a discharge circuit connected to themaznet and including a discharge resistor, 01' means to constrain saidswitching means from said normal operation when said discharge resistoris opencircuited.

8'. The combination with a control system tor m a lifting magnet to beconnected to a source 0! power, said system including switching meansnormally operable to connect the magnet to the source of power, means todisconnect said magnet irom the source oi power, and resistance meansconnected to the magnet to limit the discharge voltage of the magnet, ofmeans operative to render said switching means inoperative ,to connectthe magnetic the source of power when said resistance means isopen-circuited.

ARTHUR L. WARD.

